In application of the rotational parts such as the spindle, the bearing is usually introduced to support and position the rotational part, by which the friction for working the rotational part can be reduced without sufficing the rated load. Conventionally, bearing for the rotational part such as a spindle is usually a ball bearing having an outer ring, an inner ring, balls or rollers, and a retainer ring, in which the balls or the rollers are running between the outer ring and the inner ring, and the fixed inter-ball spacing is kept by the retainer ring.
In the aforesaid design of the ball bearing, for the balls or rollers are used as the solid rolling media between the inner and the outer brings, all parts including the inner ring, the outer ring, the retainer and definitely the balls or rollers are all involved in the frictional motion and thus are all under frictional wearing while the spindle rotates. Hence, to hold on a satisfied operation of the spindle, a high manufacturing precision of the bearing is demanded. Yet, the service time for such a bearing is still limited, and can't meet the practical requirement. To search an answer for such a shortcoming of the conventional bearing, a fluid bearing is then provided to replace the balls or rollers with a thin layer of liquid or gas, in particular an oil membrane.
With the fluid-type oil membrane to replace the solid balls or rollers, the fluid bearing can reduce the rotational friction, provide higher precision, and control the noise and temperature. In the art, the fluid bearings are broadly classified into two types: fluid dynamic bearings and hydrostatic bearings. In this disclosure, the hydrostatic bearings are the concerned.
The hydrostatic bearings in the art are externally pressurized fluid bearings. The oil membrane is generated and served as a lubricant medium between contacting parts of the bearing. With the hydrostatic bearing applied to a spindle, the wear of the spindle can be reduced, the service life thereof can be prolonged, and only less power is needed to start the spindle. Also, the spindle supported by the hydrostatic bearing can be operated in a low cycling environment.
Importantly, it is the hydrostatic bearing that the multi-task machine integrating lathing, milling and grinding can then be possible. The reason why the hydrostatic bearing plays the critical role in the appearance of the multi-task machine is because the necking developments in managing the machining chips and bearing's pre-pressuring are resolved by introducing the hydrostatic bearings to load the spindles. In the art of conventional machining tools, the spindle is set to be light-preloaded, moderate-preloaded or heavy-preloaded according to different needs of machining. In addition, in order not to damage the high precision bearing prior to the predicted service life, the selection of the bearing would be determined in accordance with the machining loads of the equipments, which are classified into types of light cutting, moderate cutting and heavy cutting. The hydrostatic bearing is a non-contact bearing that won't shorten the predicted service life in the applicable load range, for either type of cutting, as long as the rigidity of the oil membrane is under control not to induce a membrane breakdown, or say a metal-to-metal contact, between parts of the hydrostatic bearing. It is noted that the hydrostatic bearing uses the lubricant oil for forming the membrane and the operational accuracy and the service life of the spindle are highly dependent on the rigidity of the membrane. Therefore, though the hydrostatic bearing is already superior to the conventional bearing in several manifolds, yet the issue of how to improve the axial-directional rigidity of the hydrostatic spindle is still a crucial topic in the art.
Following documents are provided to help understanding the current shortcomings in the art of the hydrostatic spindle.
As disclosed by the U.S. Pat. No. 5,462,364, the hydrostatic bearing has an elastic oil chamber for controlling the fluid flow. Even that the parameters for the chamber can be relevantly selected to provide a higher rigidity, the elasticity in the oil chamber still plays an uncertain role in promising a satisfied operational accuracy.
Further, in the U.S. Pat. No. 5,921,731, a hydrostatic spindle structure is disclosed to have a special symmetric oil chamber. Though the spindle can run at high speed, yet the rigidity for the oil membrane is yet to be improved. Due to the manufacturing difficulty in structuring the symmetric oil chamber, so an acceptable operational accuracy of the spindle and a higher rigidity of the membrane are still way to be achieved.
In the U.S. Pat. No. 6,367,977, a ball bearing is introduced to enhance the axial-directional rigidity for the spindle. Though the rigidity might be increased, yet the involvement of the ball bearing would be highly possible to reduce the operation accuracy and the service life.
In all of the aforesaid disclosures, it is obvious that the rigidity of the hydrostatic bearing is determined by the area of the oil chamber, the flow resistance caused by the oil seal and the operation of the flow regulator. Therefore, the present disclosure is provided to improve the bearing rigidity of the hydrostatic spindle.